Originally Posted by letiole
Actually, there are procedures in place with ATC for determining whether or not the alarms are real. They do happen sometimes and generally don't result in diversions. Someone said this plane continued to squawk 7500 - the hijack code - so that's likely what led to the decision to force it to land. ATC likely asked the pilots to confirm, they negated, but continued to squawk 7500, leaving ATC to believe they were possibly being hijacked.
The Mode C transponder which sends this code is very old technology. Typically you have four rotary knobs with the digis 0 thru 9 on them. The VFR (no flight plan, go wherever) code is 1200. You're supposed to leave it on 1200 untul you are assigned a specific code (squawk) by a controller, then you leave it there until you are a VFR flight not using flight following, you land, or until you are hijacked, your voice radio fails, or you have some other general emergency. Then you are SUPPOSED to turn the LAST two digits FIRST, then the first two. Why?
Say you're squawking 1200. You are assigned 4649. You turn the first two dials through to 4 and 6 but pass 7 and 5 on the way around. If the transponder is polled by the radar at the exact wrong moment, you will send 7500. Alarms go off at ATC facilities, all kinds of nonsense breaks loose. THe controller, if he is in contact with you, will ask you one question and one question only, and it is words very close to "Did you really mean to squawk 7500" or simply "Confirm squawk 7500". If you say "yes" then you get pretty much whatever you want in the way of clearances, but you had darn well better have an unwelcome visitor in the cockpit when you land and you will most likely have Major Dwight Smiley on your wing the rest of the way down. If you say NO and indicate some malfunction you will probably still be escorted in these heady days of overblown security, but it's probably better that way. As a plot, I just set the last two numbers first.
--Paul